It is known from WO-A-2007/051959 to provide ultrasonic detectors which may be used to perform non-destructive testing. One desirable type of testing is to detect a change in a surface of a wall. Such a wall may, for example, be the wall of a pipe containing a fluid, such as a corrosive fluid or a multi-phase fluid. Such fluids may corrode or erode the inner surface of the wall and it is desirable to monitor such changes in the thickness of the wall or changes in the roughness of the inner wall. By monitoring in this way, potential failures and risk can be identified before a problem arises. As an example, using monitoring techniques to track the corrosion or erosion of the inner surfaces of pipes in a refinery may permit the safe refining of oil which would otherwise be regarded as too difficult due to the way in which it corrodes or erodes the pipes of the refinery.
A problem with the above type of measurement is that the inner surface of the pipe may be corroded or eroded to form a rough surface which has the effect of scattering the ultrasonic vibrations being used to measure the wall thickness in a manner which results in unacceptable inaccuracy in the wall thickness measurements. A discussion of such rough surface scattering may be found in “Application Of The Distributed Point Source Method To Rough Surface Scattering And Ultrasonic Wall Thickness Measurement” by Jarvis and Cegla, Journal of the Acoustical Society of America 132(3), September 2012, pages 1325 to 1335.